Plastics have found widespread use as a substrate material in numerous and diverse settings. Amongst other reasons, plastics are generally light weight, have high ductility and offer a high degree of visible light transmission. Certain plastics, such as polycarbonate (PC), have the added benefit of high impact strength. Applications for plastic substrates include but are not limited to automotive windows, headlamps and body panels, architectural windows, displays, solar cells and collectors, aircraft windows and canopies, and appliances. In most applications, the plastic substrate is provided with one or more functional coatings. For example, automotive windows require, at a minimum, both ultraviolet (UV) filtering coatings to protect them from exposure to sunlight, and abrasion resistant coatings to protect them from scratching. Additionally, automotive windows desirably have infra-red (IR) reflective coatings, transparent conductive coatings for heater grids, and/or electro chromic coatings. For displays, solar cells and dual-pane windows, barrier coatings to oxygen and water are important.
While providing requisite functionality, coatings oftentimes have intrinsic characteristics that ultimately prove detrimental to the final layered article. For example, many coatings have modulus values and coefficients of thermal expansion (CTE) that are significantly different than those of the underlying plastic substrate. This mismatch in properties can cause large strains in the coatings and at the layer interfaces during periods of thermal cycling and exposure to high humidity or water immersion. This in turn leads to delamination and/or cracking of the coatings.
Several approaches to address this problem have been developed. One response has been to provide compliance between the substrate and the coatings by using a graded interface. For example, U.S. Pat. No. 4,927,704 discloses formation of a graded interface by plasma enhanced chemical vapor deposition (PECVD) to provide compliance. In this approach, vinyltrimethylsilane (VTMS) or hexamethyldisiloxane (HMDSO) is used and the properties are gradually graded from that of the substrate to that of the coating. While helpful, this approach can only effectively be used in a slow deposition rate process. For low cost, high deposition rate processes that are commercially favored, this methodology is not economically practical.
Another attempt to obtain compliance has been to use single bond layers. For example, U.S. Pat. No. 5,156,882 discloses the use of organosilicones of the general formula R1nSiZ(4-n) as described in U.S. Pat. No. 4,224,378, and R2Si(OH)3 as described in U.S. Pat. No. 4,242,381. Included among specific compounds contemplated are hexamethyldisilazane (HMDZ), HMDSO, VTMS and octamethylcyclotetrasiloxane (D4).
In U.S. Pat. No. 5,718,967 a laminate is disclosed where the first adhesion promoter layer is a plasma polymerized organosilicon polymer of dimethoxydimethylsilane (DMDMS), methyltrimethoxysilane, tetramethoxysilane, methyltriethoxysilane, diethoxydimethylsilane, methyltriethoxysilane, triethoxyvinylsilane, tetraethoxysilane, dimethoxymethlyphenylsilane, phenyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, diethoxymethylphenylsilane, tris(2-methoxyethoxy)vinyl silane, phenyltriethoxysilane, dimethoxydiphenylsilane, tetramethyldisiloxane (TMDSO), HMDSO, HMDZ and tetramethylsilazane.
Despite these endeavors, there is still a need for a bond layer that provides improved compliance thereby permitting more effective thermal cycling and hydrolytic stability.